allow to use customized pdf library

src/PhpSpreadsheet/Writer/Pdf/Dompdf.php

@@ -8,6 +8,18 @@
 
 class Dompdf extends Pdf implements IWriter
 {
+    /**
+     * Gets the implementation of external PDF library that should be used.
+     *
+     * @param array $config Configuration array
+     *
+     * @return Dompdf implementation
+     */
+    protected function getExternalWriter()
+    {
+        return new \Dompdf\Dompdf();
+    }
+
     /**
      * Save Spreadsheet to file.
      *
@@ -51,7 +63,7 @@ public function save($pFilename)
         }
 
         //  Create PDF
-        $pdf = new \Dompdf\Dompdf();
+        $pdf = $this->getExternalWriter();
         $pdf->setPaper(strtolower($paperSize), $orientation);
 
         $pdf->loadHtml(

src/PhpSpreadsheet/Writer/Pdf/Mpdf.php

@@ -9,6 +9,18 @@
 
 class Mpdf extends Pdf implements IWriter
 {
+    /**
+     * Gets the implementation of external PDF library that should be used.
+     *
+     * @param array $config Configuration array
+     *
+     * @return Mpdf implementation
+     */
+    protected function getExternalWriter($config)
+    {
+        return new \Mpdf\Mpdf($config);
+    }
+
     /**
      * Save Spreadsheet to file.
      *
@@ -55,7 +67,7 @@ public function save($pFilename)
 
         //  Create PDF
         $config = ['tempDir' => $this->tempDir];
-        $pdf = new \Mpdf\Mpdf($config);
+        $pdf = $this->getExternalWriter($config);
         $ortmp = $orientation;
         $pdf->_setPageSize(strtoupper($paperSize), $ortmp);
         $pdf->DefOrientation = $orientation;

src/PhpSpreadsheet/Writer/Pdf/Tcpdf.php

@@ -8,6 +8,20 @@
 
 class Tcpdf extends Pdf implements IWriter
 {
+    /**
+     * Gets the implementation of external PDF library that should be used.
+     *
+     * @param string $orientation Page orientation
+     * @param string $unit Unit measure
+     * @param string $paperSize Paper size
+     *
+     * @return TCPDF implementation
+     */
+    protected function getExternalWriter($orientation, $unit, $paperSize)
+    {
+        return new \TCPDF($orientation, $unit, $paperSize);
+    }
+
     /**
      * Save Spreadsheet to file.
      *
@@ -51,7 +65,7 @@ public function save($pFilename)
         }
 
         //  Create PDF
-        $pdf = new \TCPDF($orientation, 'pt', $paperSize);
+        $pdf = $this->getExternalWriter($orientation, 'pt', $paperSize);
         $pdf->setFontSubsetting(false);
         //    Set margins, converting inches to points (using 72 dpi)
         $pdf->SetMargins($printMargins->getLeft() * 72, $printMargins->getTop() * 72, $printMargins->getRight() * 72);